The goal is to add multiple icons into one image with an alpha channel (transparency).
Having multiple icons in one larger image is a good optimization technique to speed up website loading.
The icon we want to add might not have transparency, in which case we have add an alpha-channel manually.
We need to resize the image, to make room for the additional icon:
After resize, notice that the 'yellowish' line (=layer selection) only has the "old" size selected
Change the layer selection to match the image
Result after layer selection
Now we add a new image - you can google for an image and then "copy" it
Now "paste" it into our image
Notice that the new icon is "too big" - in the toolbox select scale and enter 26x26
... and move it to the correct place with the "move" tool from the toolbox. When you placed it in the correct location, click somewhere in image to "apply" the move
Notice that the icon has a white "background" - we want that to be transparent, so it'll match any background. We want to draw in the alpha channel (=transparency channel). Go to the channels toolbox and switch to the channels tabpage. By default all channels are selected, deselect red, green and blue (only alpha remains selected).
Now use the "eraser" tool (the red one) to erase all the white (you might want to change the Brush-size so something appropriate)
and the end-result
Now all you need to do is save the file :)